1. Field of the Invention
The present invention relates to a flux which is used when a flip chip IC or an electronic component is soldered on a substrate, and a soldering method using the same. Further, the present invention relates to a soldering paste obtained by kneading a solder powder in paste form and a soldering method using the same.
2. Related Art
A flux containing an organic acid and a halogen compound has been used so far in the soldering of a hybrid IC (HIC) and a print substrate.
However, in recent years, removal of flux has been attempted from the standpoint of environmental pollution, and the use of an unwashed flux containing small amounts of solids and having a low content of a residue or alternative aqueous washing is currently widespread. However, in the aqueous washing, washability is bad with respect to IC, such as HIC, having a connection pitch structure expected to become still finer in the future. Assuming that the washing can be conducted, leakage due to moisture absorption or the like seems to occur under severe conditions in storing IC in a vehicle when the drying is insufficient.
Meanwhile, without the washing, the flux containing the low content of the residue seems to give an adverse effect on a post-treatment such as a resin molding step or a wire bonding step.
Further, the soldering paste is generally composed of (1) a solder powder, (2) a solvent and (3) solids (rosin, activator and the like). Commercial unwashed products decrease the solids or are free from a halogen compound. However, after the soldering, the residue of the resin component such as a rosin or the like or the activator component occurs. Especially, the residue of the activator (activator itself or the reaction product) notably impairs reliability.
On the other hand, the residue of the resin such as a rosin is not so problematic in the soldering of the print substrate (for domestic use or for a vehicle). However, in the soldering of HIC, a soldering paste in which such a residue remains even in a small amount involves problems such as terminal leakage of a flip chip, corrosion of a circuit and reliability of wire bonding in the post-treatment. Accordingly, alternative flux washing or aqueous washing is conducted at present.
Recently, a soldering method in which no residue remains by causing a soldering paste to reflow using an alcohol has been proposed in Japanese Laid-Open Patent Application Nos. 2-290693 or 2-25291.
Japanese Laid-Open Patent Application No. 2-290693 discloses a paste obtained by mixing a solder powder with an alcohol of which the boiling point is higher by approximately 30.degree. C. than a melting point of a solder, such as a monohydric alcohol, a polyhydric alcohol or an ether. The present inventors have confirmed on the basis of the above-mentioned document that with an ether and a monohydric alcohol, no solder wettability is exhibited and no solder is spread in a non-reductive atmosphere (atmosphere in which a metallic oxide is not reduced). Further, in Examples using a polyhydric alcohol such as triethylene glycol, the alcohol has to be added in quite a high amount (approximately 15% by weight), and this is inappropriate in a printing paste. When the soldering is conducted using this printing paste, the level of the soldering is not good. Even if the printing paste is obtained using triethylene glycol in an amount of approximately 10% by weight, the solder does not become wet (solder is not spread), so that the soldering paste does not work.
Still further, according to the observation of the present inventors through a high-temperature microscope, almost all of materials of which the boiling point is higher by 30.degree. C. than the melting point of the solder are evaporated before the solder is melted, the surface tension of the soldering cannot be decreased enough, and the solder is not spread.
In Japanese Laid-Open Patent Application No. 2-25291, the soldering is conducted in a reductive atmosphere to improve solder wettability, and to this end, a temperature of a reflow oven has to be as high as from 300 to 350.degree. C. or higher.
That is, the soldering pastes proposed in these documents can be expected to be materials in which no residue remains, but good wettability cannot be expected in the non-reductive atmosphere or at normal temperatures.